1. Field of the Invention
The present invention relates to methods for producing silicon carbide semiconductor substrates having high resistivity.
2. Description of the Prior Art
Silicon carbide exhibits many attractive electrical and thermophysical properties for the fabrication of high power density solid state devices operating at microwave frequencies. Silicon carbide based microwave transistors and integrated circuits may provide approximately five times the power density of gallium arsenide MMICs at X band, and approximately ten times the power density of silicon at UHF to S band frequency.
Compared to mature silicon and GaAs device technologies, silicon carbide is a relatively new technology. However, recent advances in the growth of large diameter silicon carbide substrates and the realization of silicon carbide's superior temperature, thermal conductivity, and electric field breakdown properties have prompted intensive research efforts to develop silicon carbide based electronic materials and devices extensively throughout the industry.
The silicon carbide devices currently being produced have power gains limited well below their theoretical performance by parasitic conduction and capacitive losses in the substrates. Calculations indicate that 1500.OMEGA.-cm substrate resistivity represents a minimum threshold value to achieve RF passive behavior. Also, 5000.OMEGA.-cm resistivity is needed to minimize device transmission line losses to 0.1db/cm or less. To ensure device isolation and to minimize backgating effects, resistivities approaching semi-insulating behavior (in the range of 5.times.10.sup.4 .OMEGA.-cm or higher) are the goal.
Efforts date back at least as far as U.S. Pat. No. 2,854,364 to Lely for producing device quality silicon carbide.
Many efforts have been made in recent years to produce large, device quality single crystals of silicon carbide. One attempt at growth by sublimation is described in U.S. Pat. No. 4,866,005 to Davis et al. Davis et al. do not disclose the production of highly resistive silicon carbide substrates.
It is known that impurities can be introduced as dopants into semiconductor materials. These incorporated elements give certain properties such as electrical conductivity and conduction type to the semiconductor, respectively. Furthermore, it is known in the art that impurities can be added to some other semiconductor materials to obtain high resistivity characteristics. For example, chromium is doped (incorporated) into gallium arsenide (GaAs) to achieve semi-insulating behavior. However, techniques used to produce semi-insulating behavior in GaAs are not applicable to silicon carbide, since they rely on introducing the dopant (chromium) by addition to a melt of liquid GaAs at relatively low temperatures (the melting point of GaAs 1238.degree. C. or lower, compared to the growth temperatures for SiC in excess of 2000.degree. C.). Incorporation of the chromium relies upon the high diffusivity of the impurity in the liquid GaAs and impurity segregation effects between the solid and liquid phase during solidification. These effects are not applicable to SiC since no liquid SiC state exists at practicably realizable pressures. (SiC sublimes directly from the solid phase without passing through a liquid state.) Furthermore, the exceedingly low diffusion coefficients of impurities in SiC prohibit incorporation of impurities by diffusion directly into the SiC solid.